1. Field of the Invention
This invention relates to a heat-treatment apparatus to be used for various heat-treatment processes conducted on an object of heat-treatment such as a semiconductor wafer.
2. Description of the Related Art
A heat-treatment apparatus is typically used for various heat-treatments of semiconductor wafers including those of forming an oxide film on the surface of the wafer, forming a thin film over the surface of the wafer by CVD method and producing an impurity-dispersed region on the wafer by a thermal dispersion method. A typical known heat-treatment apparatus comprises a heat-treatment section for subjecting the heat-treatment to semiconductor wafers in a predetermined manner and a loading section through which a wafer boat containing a plurality of the semiconductor wafers is loaded into and unloaded from the heat-treatment section. A vertical type heat-treatment apparatus is popular. In the vertical type heat-treatment apparatus, the loading section is normally disposed under the heat-treatment section and the wafer boat is moved up and down vertically in the loading section.
The heat-treatment section comprises a process tube made of a heat resistant material such as quartz, a heater disposed around the process tube for heating the wafers and gas supply means for supplying process gas to the process tube. The wafer is heat treated at a predetermined temperature in a process gas atmosphere within the heat-treatment section.
The loading section comprises a support member for supporting the wafer boat and a drive mechanism for driving the support member to move to a direction of the loading/unloading the wafer boat. The drive mechanism is typically provided with a ball shaft which is arranged in the direction of loading/unloading the wafer boat and functions as a male screw of a ball screw. Onto the ball shaft, a ball nut formed at an end of the support member functioning as a female screw of the ball screw is connected to engage each other. An end of the ball shaft is connected to the drive shaft of the motor. With such an arrangement, the shaft is driven by the motor so that the support member is vertically moved as the ball shaft and the ball nut interact with each other. As a result, the wafer boat is loaded into or unloaded it from the process tube.
In recent years, a semiconductor device tended to be highly integrated and hence sophisticated. In a manufacture of such a device, the yield of manufacturing of it is significantly reduced by fine particles that adhere to the surface of wafers during the heat-treatment process. Hence, wafers are required to be handled under very clean conditions.
However, the above described drive mechanism involves mechanical motions of some or most of its components, which are by turn inevitably subjected to wear. When, for example, the ball shaft and the ball nut are operated for relative movement, they are worn, if slightly, to produce fine particles that come off from the surfaces thereof. Hence, it is very important to protect the surface of the wafers in the wafer boat against such particles that may adhere to it.
The wafers and the wafer boat unloaded from the process tube normally show a temperature between approximately 150.degree. and 250.degree. C. This means that the drive mechanism receives radiation of heat from the wafers until they are sufficiently cooled by clean air and the radiated heat can deteriorate, if partly, a lubrication oil of the drive mechanism until particles of degraded oil are scattered around.
The loading section is, on the other hand, provided with a load-lock chamber. Any gas in the chamber can be evacuated to vacuum and the inside of the chamber can be purged with an appropriate gas such as nitrogen gas in order to prevent the heated surface of the wafers from being oxidized.
A heat-treatment apparatus is additionally provided with a wafer stocker section for temporarily storing wafers and a wafer transfer section for transferring wafers from the wafer cassette to the wafer boat. Wafers are moved from the wafer cassette to the wafer boat on a one by one basis or in batches of a given number of wafers. It is, however, extremely difficult for the mechanism of transferring wafers employed in the wafer transfer section to accurately align all the wafers on a wafer boat in terms of the orientation flats of the wafers. Therefore, when the wafers are subjected to the heat-treatment within the process tube, there can arise gas turbulences in it, making the condition surrounding the surfaces of the wafers in the tube remarkably uneven and disadvantageous for heat-treatment and reducing significantly the yield of manufacturing wafers in such an apparatus.
The wafer loading robot of the wafer loading section can also be adversely affected to eventually go out of function by the heat radiated from the heat-treatment section of the apparatus. In an attempt to prevent any such malfunction of the robot, a robot chamber is normally provided in the wafer loading section to contain a wafer transferring mechanism such as the robot, and equipped with gate valves which connect the robot chamber and the load-lock chamber or the wafer stocker section. The gate values are normally kept closed to block the heat radiated from the load lock chamber and is opened only when wafers are transferred from the wafer cassette to the wafer boat.